Ceramic materials are commonly prepared by mixing powdered ceramic oxides such as magnesia, alumina, titania and zirconia, in a slurry along with additives, such as dispersants and binders. The slurry may be spray dried to produce ceramic particles. The particles are pressed into an aggregate structure, called a "green ceramic," having a desired shape and subsequently subjected to a severe heat treatment known as sintering. The sintering process converts the green ceramic into a cohesive "fired ceramic", having a nearly monolithic polycrystalline ceramic phase.
The binder serves to hold the ceramic particles of the green ceramic in the desired shape after pressing. The binder can also provide lubrication while the particles are pressed. Preferably, the binder combusts or vaporizes completely during the sintering process leaving no trace of the binder in the fired ceramic. In performing these functions, binders significantly affect the properties of the fired ceramics which are ultimately produced.
In commercial practice, poly(vinyl alcohols) are widely used as ceramic binders. Additionally, poly(ethylene oxide) and ethylene-vinyl acetate copolymers reportedly have been used as binders for particulate material, such as granular silica gel.
For example, polymeric binders containing substantially hydrolyzed copolymers made from monomers having ester or amide functional groups, poly(vinylformamide) or a copolymer of vinyl alcohol and vinyl amine are disclosed in U.S. Pat. Nos. 5,358,911; 5,487,855 and 5,525,665.
Furthermore, polymeric treatments have been disclosed in U.S. Pat. Nos. 4,680,339; 4,731,419; 4,885,345 and 5,084,520. Utility for the treatments has been disclosed to be as dispersants in water treatment, scale inhibitors in industrial and natural waters, flocculants, coagulants and thickeners; but ceramics applications of binding and dispersancy have not been disclosed.
Although commercially available binders are satisfactory for many applications, a need exists for improved binders which provide still greater strength and/or increased density in green ceramic materials. Greater green strength reduces breakage during handling of the green ceramics and, generally, is associated with higher quality fired ceramics. Preferably, the improved binders would be cheaper and more versatile than previously known binders.
Spray drying is an evaporative process in which liquid is removed from a slurry containing a liquid and a substantially non-volatile solid. The liquid is vaporized by direct contact with a drying medium, usually air, in an extremely short retention time, on the order of about 3 to about 30 seconds. The primary controlling factors in a spray drying process are particle size, particle size distribution, particle shape, slurry density, slurry viscosity, temperature, residence time, and product moisture.
The viscosity of the slurry must be suitable for handling and spray-drying. Although spray-drying equipment conditions may be adjusted to handle a variety of viscosities, larger particles will usually result from higher viscosity slurries.
Those of ordinary skill in the art are familiar with the spray-drying process used in the production of ceramic materials, and will be able to optimize the control factors of spray-drying to best advantage. Alternatively, the spray drying process may be replaced by other well known drying methods, such as granulation, tape casting and slip casting.
Spray drying of the slurry produces substantially dry, free-flowing powder particles which contain the ceramic, the binder and the optional materials described above. The dry particles are granules which are generally spheroidal in shape and have an effective diameter of about 50 to about 300 micrometers. Typically, about 0.5 percent to about 8 percent of the binder, based on the dry weight of the ceramic powder, is present in the dry particles.
In granulation, a mixture of dry powder or powders is mixed or rolled, commonly in a barrel shaped apparatus. Water and/or a binder solution is sprayed into the mixing powder causing aggregation of the small particles into larger granules. The size of the granules is controlled by the amount of material sprayed into the powders and the speed with which it is sprayed. Granulated powders may be screened to a desired size and pressed to shape in a pressing operation prior to sintering. Alternatively, the granules themselves may be the desired product and may be sintered directly.
Tape casting is commonly used to produce thin substrates for the computer industry. In the process, a thick ceramic slurry containing ceramic powder, dispersant and binders is prepared. This slurry is cast onto a smooth surface such as a Mylar or plastic sheet and the thickness is controlled by passing the sheet under a blade which smoothes the slurry surface and scrapes off excess material. The slurry tape is dried to a plastic state and cut and shaped to specification. The amount of binders present in tape casting is very high, typically on the order of 15 to 20 wt. % of the ceramic powder mass.
In fluidized bed spray drying, small "seed" particles are placed in a column and hot air is blown into the seed powder from below suspending the particles in the column. A ceramic slurry is sprayed onto the seed particles from above, causing them to grow. When the particles reach a large enough size, they are siphoned out of the dryer while more seed particles are introduced. This process can produce powder for further forming processes, or the powder itself may represent the desired product, in which case it would be sintered to produce the final ceramic.
The dry particles are compacted to produce an aggregate, green ceramic structure. Preferably, the particles are compacted by pressing in dies having an internal volume which approximates the shape desired for the final fired ceramic product. Alternatively, the particles are compacted by roll compacting or other well-known compacting methods. The spray dried blend of powder, binder, and optional surfactants and lubricants is relatively free flowing so that it can enter and closely conform to the shape of the pressing dies.
Inside the dies, the dry particles are subjected to a pressure which is typically in the range of about 5000 to about 50,000 psi. Pressing the particles produces an aggregate structure, called a green ceramic, which retains its shape after removal from the die.
One forming technique used for spray dried or granulated material is roll compaction, also referred to as roll pressing. This technique takes a dry powder and crushes it between two rollers in a continuous process. This process produces sheets of ceramic of various widths and thicknesses. These sheets can be cut to shape and sintered to produce the final ceramic body. The process is commonly used to produce ceramic substrates for the electronics industry.
Dry pressing involves filling a shaped die with spray dried or granulated powder and pressing it at high pressures. The pressing occurs through movable pistons at the top and/or bottom of the die cavity. The process can be used to produce fairly complex geometries in a single forming step. The ceramic body that results is ejected from the die and sintered to produce a final ceramic product.
Isostatic pressing is similar to dry pressing in that a ceramic powder is pressed in a die cavity. In isostatic pressing, however, all or part of the die wall consists of a flexible material. After filling the die cavity with powder, the die is submerged in a liquid pressure chamber and pressure is applied to squeeze the die and compact the powder. Unlike dry pressing, no movable parts are involved. Isostatic pressing is commonly used on large or very long parts to minimize cracking or lamination of the final ceramic green body.
Extrusion involves the pushing of a concentrated, plastic, slurry through an orifice. The orifice is of the size and shape of the desired ceramic body. This process is commonly used to produce ceramic tubes or similarly shaped pieces. The slurry used is prepared from dry powders which are mixed with water, organic binders and lubricants, and a coagulant. This slurry is usually predried in a filter press or similar apparatus to remove excess water and thicken the slurry to a plastic material. The material is then extruded through a press which is either piston or screw driven. The extruded material is cut to length, dried, and sintered.
Jiggering is commonly used in the whiteware industry to shape an extruded or filter pressed ceramic slurry. Typically, a portion of the plastic slurry is placed on a rotating wheel and shaped by rollers and/or knife blades to a desired geometry. This body is then dried and sintered.
Another ceramic forming method, that is used for parts of complex shape, is slip casting. In slip casting, a concentrated ceramic slurry (slip) is poured into a mold with an internal shape of the desired ceramic body. The slurry used must be highly concentrated to prevent settling of particles and/or excessive shrinkage during drying. At the same time, the slip must be fluid enough to completely fill the mold and allow escape of air bubbles. The presence of a polymeric binder adds strength to the cast body preventing breakage during mold removal and handling of the body prior to sintering.
Heating the aggregate structure drives off volatile materials such as water, and bums off organic materials, such as binders or surfactants. When a sufficiently high temperature is reached, the particles of the aggregate structure begin to fuse, but do not fuse completely, and become fastened to one another to reproduce a relatively strong fired ceramic material having essentially the desired shape.
The slurry is, for example, spray dried to produce substantially dry particles. The particles are preferably pressed to produce an aggregate, green ceramic structure and heated to produce a fired ceramic material. Alternatively, the particles can be formed into an aggregate, green ceramic structure by roll compaction or other well-known methods.
Although commercially available binders are satisfactory for many applications, a need exists for improved binders which provide still greater strength and/or high density in green ceramic materials. Greater green strength reduces breakage during handling of the green ceramics and, generally, is associated with higher quality fired ceramics. Preferably, the improved binders would be cheaper and more versatile than previously known binders.
The present invention also relates to a method for dispersing ceramic materials. In particular, the present invention relates to a method for dispersing one or more ceramic materials in an aqueous medium by using a polymeric dispersant formed from acid-containing monomers and hydroxy functional monomers.
Ceramic materials are often used to prepare lightweight, strong, thermally and chemically resistant products. Because of difficulties associated with the handling of solid ceramic materials, it is desirable for the ceramic materials to be in the form of an aqueous dispersion. Aqueous dispersions of ceramic materials are, however, often unstable, exhibiting sediment formation upon standing. Upon standing, the dispersion agglomerates and becomes non-homogeneous, and creates difficulty in handling. These agglomerates may also damage pipes, pumps, and other dispersion handling mechanical equipment. The use of dispersants overcomes these difficulties, and also improves strength and density of formed ceramic parts, particularly those made by dry press, slip casting, and tape casting processes.
Polymers are known for use as dispersants for ceramic materials. Typical polymeric dispersants for ceramic materials include polymers formed from acid-containing monomers such as, for example, poly(acrylic acid) and poly(methacrylic acid). For example, anionic polymers produced by hydrolyzing a terpolymer of maleic anhydride, N-vinylpyrrolidinone and a vinyl compound selected from the group consisting of acrylic acid, acrylamide, methyl methacrylate and butyl vinyl ether is disclosed in U.S. Pat. No. 5,266,243. Additionally, polymeric dispersants consisting of from 5 to 95 percent by weight of one or more hydroxy functional monomers and from 95% to 5% by weight of one or more acid-containing monomers are disclosed in U.S. Pat. Nos. 5,567,353 and 5,532,307. The hydroxy functional monomer is selected from the group consisting of hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, allyl alcohol, allyloxyethanol, allyl propoxylate, vinyl acetate, 1-butene-3,4-diol and 3-allyloxy-1,2-propane diol.
Furthermore, imidized acrylic polymers have been disclosed for the increase of flowability in cement compositions in U.S. Pat. No. 5,393,343.
While such polymers perform adequately in dispersing some ceramic materials, certain ceramic materials are more difficult to disperse, and conventional polymeric dispersants are not adequate. Ceramic materials which present particular difficulty in forming dispersions include nitrides such as, for example, boron nitride. U.S. Pat. No. 5,209,885 describes dispersing silicon nitride for extrusion by the use of a graft copolymer comprising a polyoxyalkylene backbone with polyacrylate side chains.
The present invention seeks to provide a method for dispersing ceramic material, including several ceramic materials known to be difficult to disperse.